Enhancing communication to the user whether interpolation settings meet conditions for default grid

This MR tackles Issue 70 in which the export to *.pol is disabled and left the users of StressSpec cluecless why.

After I've digged long and deep through the sourcecode and looked through the previous release versions, I think this issue is mostly a communication problem hence my solution to this.

Allow me to recap what has happened three years ago: A series of changes has been made to certain parts of the source code which allows/disallows the export to pol files. These conditions are the following:

• user must toggle on interpolation
• step alpha and step beta values must be equal
• their values must be either 1,2 or 5 deg

As long as these conditions are met the export function works. I coudldn't find any other problem with this or any other side effect. Therefore I think the most conservative thing is to just increase the communication about this to the user. For this purpose I've changed the ToolTip text which is displayed to make this more clear (also disable the ToolTip if it's not needed). Additionally I've added a read-only indicator (checkbox) on the Interpol tab to show if the conditions are met to enable the pol figure export.

I was also thinking about adding more documentation text to the GUI but in the end it was just overblown and I think communication and documentation should be enough for this.

In case you have found further information about this change three years ago (which may or may not change the reasoning here) then please let me now. But from what I've seen so far I think this solution makes the most sense.

Also if you have further question we can talk via slack or phone.

Also we need to integrate this in our documentation (I'm working currently on that too, as I've mentioned) but also we need to communicate this to StressSpec (and maybe Salsa?) with a little demonstration.

Resolves #70 (closed)